Optical storage systems typically use a radiation beam generated and processed in an optical head to record data on and/or retrieve data from an optical storage medium. Many of these systems utilize differential detection in order to detect small signal fluctuations in the presence of various types of system noise. An example is a conventional magneto-optic (MO) system, in which data is stored on an MO medium in the form of marks having a distinct magnetic orientation. MO systems generally utilize Kerr rotation of a return beam reflected from the MO medium to distinguish marked and unmarked areas. The Kerr rotation produces relatively small variations in the return beam and is therefore difficult to detect without differential detection. Differential detection channels are provided in the MO system by separating the return beam into two orthogonal polarization components using a polarization beam splitter. The components are applied to separate detectors, and the resulting detected signals are applied to inputs of a differential amplifier which generates a differential MO data signal representative of the stored data.
In differential detection channels, it is usually important to maximize the common-mode noise rejection in order to ensure optimal system performance. Significant degradations in output data signal carrier-to-noise ratio (CNR) may result if, for example, one or more of the elements in the differential channels do not provide substantially equivalent gain and/or phase variations. One available technique for avoiding such degradations involves imposing strict tolerances on the alignment and/or performance of various system elements including polarization beam splitters, waveplates, detectors and amplifiers. However, such strict tolerances can significantly increase optical head design and manufacturing costs, and may result in a head which is unduly susceptible to, for example, variations in media parameters. Furthermore, it is generally difficult to maintain such tolerances over a broad range of operating frequencies and/or temperatures.
Another possible approach involves utilizing variable gain components in one or more of the differential detection channels to reduce amplitude differences between the detected signals at the differential amplifier input. U.S. Pat. No. 4,691,308 discloses an MO system with differential detection channels and a variable gain amplifier in one of the channels. The variable gain in one channel is adjusted in response to an error signal corresponding to amplitude differences between the detected signals. The variable gain adjustment attempts to reduce the amplitude difference between the detected signals such that common-mode rejection in the differential amplifier is improved. However, this one-channel variable gain system is susceptible to a number of problems, including long-term drift in signal levels, variable phase shifts as a function of signal level, and poor recovery from non-ideal conditions such as out-of-focus or media defects. Other problems with one-channel variable gain systems include the inability to adequately compensate for undesirable output signal modulation resulting from, for example, media birefringence.
Japanese Patent Publication No. 4-298836 entitled "Magneto-optical Recording and Reproducing Device" appears to disclose an MO detection system which uses a pair of level control circuits controlled in accordance with "double refractivity information." However, this system does not appear to improve common-mode rejection in differential detection. Furthermore, it apparently utilizes a common control signal for both level control circuits and thus fails to solve the long-term drift, output signal modulation and other problems inherent in the one-channel variable gain system of U.S. Pat. No. 4,691,308.
As is apparent from the above, a need exists for a magneto-optic system with differential detection which provides improved common-mode rejection and output MO data signal CNR without the strict tolerances, long-term drift and other problems of the prior art.